Subterranean Organs in Plants

Subterranean Organs

• Subterranean organs are plant parts growing underground, either of root or stem origin
• Stem structures include corms, bulbs, stem tubers and rhizomes
• Root structures include true, adventitious roots, and root tubers
• They lack chlorophyll and are commonly used as storage organs in perennial plants
• They are rich in carbohydrates like starch, sugars and inulin

Differences between Subterranean Stems and Roots

• Stems have nodes and internodes while roots do not
• Stems have scale leaves and axillary buds, roots do not
• Stems have mainly transverse wrinkles, roots have mainly longitudinal wrinkles
• Stems contain wide pith in the center, roots have a solid mass of xylem vessels without pith
• Stems have pericycle, roots do not

The Root

• The root grows vertically downward into the soil, away from light
• It lacks nodes, internodes, leaves and buds
• It's growing point is covered by the root cap
• Root branching is described as endogenous, originating from the pericycle
• Dicot roots develop into a tap root system, while monocot roots develop into a fibrous root system
• The root is composed of four zones: the growing point, zone of elongation, zone of root hairs and zone of lateral branches

Root Functions

• Absorption of water, minerals and organic compounds from the soil
• Anchoring the plant in the ground
• Serving as a storage organ
• Propagation

Types of Roots

• Primary roots develop into tap roots (e.g., Senega, Belladonna)
• Secondary roots are lateral roots (e.g., Krameria)
• Adventitious roots originate from the stem (e.g., Ipecacuanha)
• Storage roots are swollen with reserve food materials
• Tuberous roots are very swollen, e.g., Jalap

Histology of the young root (primary root)

• Piliferous layer: single row of thin-walled cells lacking stomata, cuticle and intercellular spaces, with root hairs
• Cortex: wide and parenchymatous

Radix Glycyrrhizae (Liquorice)

• The dried peeled or unpeeled root and rhizome of Glycyrrhiza glabra variety typica (Spanish Liquorice)
• G. sources grow in Spain, France, England, Italy, Germany and USA
• Stolons are horizontal underground stems that produce new plants from their nodes

Spanish Liquorice

• Stolons with few pieces of roots, cylindrical shape, unbranched
• Unpeeled color: dark brown or reddish-brown; peeled color: yellow
• Fracture is fibrous, internally bright yellow
• Stolons bear small buds, scaly leaves, and scars of slender side roots
• Roots bear small rootlet scars
• Peeled liquorice has very sweet taste, unpeeled has a sweet taste followed by bitterness

Microscopical Characters

• Liquorice rhizomes and roots have typical structure except for the absence of pith and pericycle in the root
• The root has tetrarch primary xylem in the center
• The rhizome shows layers including cork, phelloderm, pericycle, phloem, cambium, secondary xylem, medullary rays and pith
• The peeled liquorice lacks cork, cortex and sometimes part of the phloem

Powder

• Light-yellow in peeled, brownish-yellow in unpeeled
• Faint characteristic odour and very sweetish taste
• Microscopical features include starch granules, prismatic crystals of calcium oxalate, cork cells, fibres, vessels, fibres of pericycle, phloem, xylem, wood parenchyma and medullary ray cells

Active Constituents

• Triterpenoidal saponins: glycyrrhizin (5–7%), a glycoside 50 times sweeter than sugar
• Glycyrrhein: very sweet taste, hydrolysis results in glycyrrhetic acid (loses sweet taste) and 2 molecules of glucuronic acid

Uses

• Expectorant: loosens and thins mucus
• Demulcent: relives internal membrane inflammation
• Antiulcerogenic activity: used in treatment of stomach and deudenal ulcers
• Glycyrrhetic acid has deoxycorticosterone effect: used in treatment of rheumatoid arthritis, inflammatory conditions and Addison's disease
• Flavonoid components: antimicrobial and antiviral activity (externally as a gel)
• Glycyrrhetic acid has hormonal activity: estrogenic action, used as a starting material for semi-synthesis of cortisone and sex hormones
• Prevent liver toxicity: through membrane stabilization, inhibiting viral C multiplication and inducing interferon production

Two Types of Liquorice

• "Standard" liquorice
• "De-glycyrrhizinated" liquorice (DGL): glycyrrhizin is removed
• Each type is suitable for different conditions

Uses

• Expectorant: effectively liquefies mucus
• Demulcent: relieves internal membrane inflammation
• Hyperacidity: removes irritating effects of acids

Antiulcerogenic activity

• Deglycyrrhizinated liquorice (DGL): useful for treating ulcers due to flavonoids that kill Helicobacter pylori

Deoxycorticosterone effect

• Glycyrrhizin's action is similar to adrenal cortex hormones, especially desoxycorticosterone, used to treat rheumatoid arthritis, inflammatory conditions and Addison's disease

Antimicrobial and antiviral activity

• Flavonoid components have antimicrobial and antiviral activity (externally as a gel)

Side Effects and Contraindications

• Excessive liquorice consumption can lead to "mineralocorticoid syndrome"